The present invention relates to current drive systems, and particularly relates to techniques with current drive systems suitable as display drivers for organic EL (Electro Luminescence) panels.
In recent years, flat panel displays such as organic EL panels have grown in size and definition and have become thinner, lighter and less expensive. In driving large and high-definition display panels, an active matrix type is preferably chosen in general. Hereinafter, a conventional display driver for an active matrix type display panel will be described.
FIG. 10 shows a circuit configuration of a current drive system as a conventional display driver connected to a display panel. A current drive system 100 includes: m drivers 11-1 to 11-m for current-driving respective display element circuits 21-1 to 21-m in a display panel 20; and a bias circuit 12 for generating a bias voltage Vb and supplying the bias voltage Vb to the driver 11-i (where i is an integer from 1 through m). The display panel 20 is an organic EL panel.
The bias circuit 12 includes: a current mirror circuit 123 having p-type transistors 121 and 122 at its input and output sides, respectively; a resister 124 connected to the p-type transistor 121 and allowing a reference current Iref to flow at the input side of the current mirror circuit 123; and an n-type transistor 125 connected to the p-type transistor 122, receiving a mirrored bias current Ib at the output side of the current mirror circuit 123 to generate the bias voltage Vb.
The driver 11-i includes: n n-type transistors 111-1 to 111-n; and switches 112-1 to 112-n associated with the respective n-type transistors 111-1 to 111-n. For example, if n is 63, the driver 11-i is capable of producing a display of six bits, i.e., 64 levels of gray scale.
The gates of the n-type transistors 111-1 to 111-n in the current drive system 100 are connected to each other through a bias line 13 extending from the gate and drain of the n-type transistor 125 in the bias circuit 12 and receive the bias voltage Vb in common. That is, the n-type transistor 111-j (where j is an integer from 1 through n) forms a current mirror circuit together with the n-type transistor 125. The n-type transistor 111-j draws a current mirrored from the bias current Ib between the source and drain thereof.
The switch 112-j is connected to an output terminal 113-i of the driver 11-i at one end and is connected to the n-type transistor 111-j at the other end. The switch 112-j performs switching operation independently of the other switches based on display data (not shown).
Specifically, the driver 11-i substantially operates as a current mode D/A converter, receives display data as a digital signal and draws a current in an amount corresponding to the display data as an analog signal through the output terminal 113-i.
Each display element circuit 21-i corresponds to one pixel in the display panel 20. The display element circuit 21-i includes: an organic EL device 211; a TFT (Thin Film Transistor) 212 connected to the organic EL device 211; and a TFT 213 forming a current mirror together with the TFT 212.
As well known in the art, an organic EL device exhibits rectification as a diode and has its luminance changed depending on the amount of flowing current. Specifically, in the display element circuit 21-i, the amount of a current flowing in the organic EL device 211 varies depending on the amount of a current flowing in the TFT 213, which is connected to the driver 11-i via a drive line 30-i. Accordingly, the organic EL device 211 is current-driven by the driver 11-i to have its luminance changed.
In this manner, the current drive system 100 current-drives the plurality of display element circuits 21-1 to 21-m in the display panel 20 based on display data, thereby producing a gray-scale display (see, for example, Japanese Laid-Open Publication Nos. 11-88072 and 11-340765).
However, in the case of displaying specific display data with the conventional current drive system 100, the display might be distorted by injection of charge from the display panel 20 or instantaneous variation of the bias voltage. That is, so-called display crosstalk might occur. Hereinafter, it will be described how the display crosstalk occurs.
FIG. 11 shows a state of the current drive system 100 when the current drive system 100 is induced from the display panel 20. Though all the switches 112-1 to 112-n in the driver 11-1 are OFF in FIG. 10, the switches 112-1 to 112-n are ON in FIG. 11.
FIGS. 12A and 12B show respective examples of a display on the display panel 20. A display associated with a scan line shown in FIG. 12A corresponds to the operation state of the current drive system 100 shown in FIG. 10. A display associated with a scan line shown in FIG. 12B corresponds to the operation state of the current drive system 100 shown in FIG. 11.
In an organic EL panel, during one horizontal period, display data is written into pixels (display element circuits) on a scan line and, when this write operation is completed, a next scan line is selected so that other display data is written, as in a hold-type display panel such as a liquid crystal panel. In actual application, capacitances (not shown) for holding data are provided in the display element circuits, and these capacitances hold a voltage associated with display data until the next frame is selected. This allows the display element circuit 21-i to maintain a constant luminous state even if the display element circuit 21-i is electrically separated from the driver 11-i.
In the display associated with the scan line shown in FIG. 12A, the left part of the scan line exhibits the minimum luminance (black display) and the right part thereof exhibits the maximum luminance (white display). In this case, in the current drive system 100, all the switches 112-1 to 112-n in the driver 11-1 are OFF as shown in FIG. 10, so that the amount of a current drawn from the output terminal 113-1 is substantially zero. Accordingly, the organic EL device 211 in the display element circuit 21-1 is in a nonluminous state. On the other hand, all the switches 112-1 to 112-n in the driver 11-m are ON, so that the amount of a current drawn from the output terminal 113-m is at the maximum. Accordingly, the organic EL device 211 in the display element circuit 21-m is in a luminous state with the maximum luminance.
FIGS. 13A and 13B are graphs showing IV characteristics of the driver 11-i and display TFTs. As shown in FIG. 13A, the drivers 11-1 and 11-m produce a black display and a white display, respectively, unlike the TFTs 212 and 213 which exhibit a constant IV characteristic. FIG. 13A shows that the voltage V1 at the operating point of the black-display TFT is relatively high and can be close to the power-supply voltage. On the other hand, the voltage V2 at the operating point of the white-display TFT is lower than the voltage V1 at the operating point of the black-display TFT. These operating-point voltages V1 and V2 vary depending on the ON resistances of the TFTs and the amount of a current drawn into the driver 11-i.
FIG. 12B shows an example of a display when the left part of the scan line comes to have the maximum luminance immediately after continuation of the same display as the display associated with the scan line shown in FIG. 12A. At this time, in the current drive system 100, all the switches 112-1 to 112-n in the driver 11-1 are ON as shown in FIG. 11 so that the maximum amount of current is drawn through the output terminal 113-1. In this manner, the organic EL element 211 in the display element circuit 21-1 is in a luminous state with the maximum luminance.
In this case, charge accumulated in a parasitic capacitance 31-1 is injected into the driver 11-1 through the drive line 30-1. The parasitic capacitance 30-1 is considered to be a combination of parasitic capacitances present in the current drive system 100, display panel 20 and drive line 30-1.
If the amount of charge to be injected is relatively small, the charge passes through the n-type transistors 111-1 to 111-n to reach the ground. However, since the display element circuit 21-1 had been producing a black display immediately before the state shown in FIG. 12B, the parasitic capacitance 31-1 is charged at a voltage near the power-supply voltage. Accordingly, at the moment at which the driver 11-1 and the drive line 30-1 are electrically connected to each other, a voltage close to the power-supply voltage is applied to the drain of the n-type transistor 111-i, resulting in that the bias line 13 is disadvantageously induced through a parasitic capacitance Cgd present between the gate and drain thereof. A waveform 14 shown in FIG. 11 represents a voltage variation caused on the bias line 13 by this induction.
If a rising voltage as shown by the waveform 14 shown in FIG. 11 occurs on the bias line 13, the amount of a drive current in the other drivers, e.g., the driver 11-m temporarily increases though display data does not change. As a result, as shown in the graph shown in FIG. 13B, the driver 11-m is in an overdrive state.
If the voltage variation on the bias line 13 converges within a period during which display data is written, the driver 11-m returns to a given drive state so that a normal display is produced. However, if the voltage variation does not converge within the display-data writing period, the display element circuit 21-m remains in the overdrive state until the next frame is selected, resulting in display crosstalk in which an emission line is visually recognized.
In contrast, in a case where the display driven by the driver 11-i is switched from white to black, a temporary drop of the voltage occurs on the bias line 13. This causes display crosstalk in which a dark line having decreased luminance is visually recognized.
The parasitic capacitance 31-i is in the range from several pF to several tens pF in the case of small panels for portable use, but can be 100 pF or more in the case of large panels. Accordingly, if the display panel becomes larger in size, display crosstalk is more noticeable. In particular, a current drive system for an organic EL panel drives display element circuits by a very small amount of current of about several tens nA, so that display crosstalk is liable to occur. In recent years, current drive systems serving as display drivers for flat panel displays have been required to be able to reduce variation between output terminals as well as to enhance uniformity in displayed image quality. To meet these demands, the display crosstalk should be avoided in order to enhance the uniformity in displayed image quality.